1. Field of the Invention
The invention concerns the field of reaction injection molded polyurethanes.
2. Description of the Prior Art
Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and placing the mixture into a mold were reaction proceeds. After reaction and demolding, the parts may be subjected to an additional curing step which comprises placing the parts in an ambient temperature of about 250.degree. F. Indeed, the standard industry practice has been to post cure RIM parts at 250.degree. F. The article "Processing and Properties of a Microcellular Foam System with Low Sensitivity to Temperature", Robert L. McBrayer and Gary J. Griffin, Journal of Cellular Plastics, July through August 1977, reveals temperatures up to 300.degree. F. However, the article indicates that temperatures greater than 250.degree. F. may not be practical due to part distortion. U.S. Pat. No. 4,098,773 refers to heating RIM elastomers in the reaction mold without waiting for the reaction to complete at temperatures ranging from 212.degree. F. to 392.degree. F. The patent states that preferably this curing/reacting temperature is from 212.degree. F. to 300.degree. F. However, in the only examples where parts were actually heated in this manner, temperatures of only 212.degree. and 248.degree. F. were used.
It has been surprisingly discovered that RIM polyurethane parts may be post cured at temperatures well above 300.degree. F. and that a substantial improvement in properties takes place due to the high post curing temperature.